NOx reduction system for diesel engines, using hydrogen selective catalytic reduction

ABSTRACT

An emission control system for reducing NO x  in the exhaust of a diesel engine. A partial oxidation system receives diesel fuel from the engine&#39;s fuel tank and partially oxidizes the diesel fuel into hydrogen. The hydrogen is then introduced into the main exhaust line and the hydrogen-enhanced exhaust is delivered to a hydrogen selective catalytic reduction unit, which uses the hydrogen to convert the NO x  to nitrogen.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/093,199 filed Mar. 7, 2002, now U.S Pat. No. 7,135,153 the contentsof which is hereby incorporated in its entirety by reference.

TECHNICAL FIELD OF THE INVENTION

This invention relates to emission reduction systems for diesel engines,and more particularly to nitrogen oxide reduction using ahydrogen-selective catalytic reduction catalyst.

BACKGROUND OF THE INVENTION

In an effort to reduce ambient levels of air pollution in the UnitedStates, the United States Environmental Protection Agency (EPA) hasproposed a tightening of the emissions standards for heavy-duty dieselengines. This proposal includes measures for reducing the allowablesulfur content of diesel fuel. The proposal aims to lower emissions byabout 95 percent, with nitrogen oxides (NO_(x)) and particulate matter(PM) emission standards of 0.2 and 0.01 gram per brake horsepower hour,respectively.

Existing aftertreatment technologies for achieving these goals includeboth PM reduction systems and NO_(x) reduction systems. For PMreduction, existing technologies include a continuously regeneratingtrap (CRT®) and catalyzed traps. The term “CRT®” refers specifically tothe particulate filter manufactured by Johnson Matthey of London, UnitedKingdom, described in U.S. Pat. No. 4,902,487. For NO_(x) reduction,existing technologies include selective catalytic reduction (SCR)systems that use urea as the reductant, and NO_(x) storage catalysts.

Various factors determine which aftertreatment technology is mostsuitable for diesel engine exhaust. One consideration is the effect ofthe sulfur content in the diesel fuel. Sulfur increases the regeneratingtemperature of a CRT, which adversely affects its performance. Sulfur isalso a poison for NO_(x) traps. Because of the negative effects ofsulfur on aftertreatment performance, the EPA is recommending a dieselfuel sulfur cap of 15 ppm.

However, evidence implies that 15 ppm may still be too high for NO_(x)traps to be effective. As a result, urea SCR systems may be a moreeffective method for adequate NO_(x) reduction.

Despite their effectiveness, urea SCR systems are not without theirshortcomings. Urea SCR is based on ammonia reduction, with urea beingthe reductant of choice for vehicular applications, due to theperception that a supply of ammonia on-board a vehicle would be unsafe.Ammonia is considered to be highly toxic, whereas urea is only mildlytoxic. But the problem with urea SCR is that a separate supply of ureais required on-board. Not only does this requirement call for a separatestorage tank, but the urea must be replenished periodically and there isno infrastructure to provide a nationwide supply. Also, the systemrequired to introduce urea into the exhaust stream is complex. In sum,there are many issues affecting the practicality of using urea for SCR.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a first embodiment of the invention.

FIG. 2,is a block diagram of a second embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention described herein is directed to an SCR system that doesnot require urea as the reductant. The system uses diesel fuel insteadof urea, which eliminates the requirement for a second supply tank andthe need for a urea supply infrastructure.

FIGS. 1 and 2 describe two different embodiments of the invention. Bothuse an oxidation unit 10 a and a hydrogen selective catalytic reduction(H-SCR) unit 10 b, but in different configurations. Both embodiments areused with diesel engines, which do not run rich. The oxidation unit 10 aacts as a hydrogen generator, and runs “offline” of the main exhaust gasstream so that it may operate in a rich fuel environment. The hydrogenfrom oxidation unit 10 a is fed to the H-SCR catalyst 10 b, whichcontinuously converts NO_(x) to N₂ and H₂O. An optional water gas shift(WGS) catalyst 10 c may be interposed between the partial oxidation unit10 a and the H-SCR catalyst 10 b, to generate additional hydrogen.

As explained below, diesel fuel is partially oxidized by oxidation unit10 b to produce a combination of hydrogen (H₂) and carbon monoxide (CO),with traces of carbon dioxide (CO₂) and water (H₂O) produced as byproducts. The hydrogen is then used by an H-SCR catalyst 10 b to convertthe NO_(x) in the exhaust stream into nitrogen. The H-SCR catalyst 10 bis selected specifically to use hydrogen to reduce exhaust-borne NO_(x)emissions, and operates under net oxidizing conditions (lambda>1).

FIG. 1 illustrates one embodiment of an H-SCR (hydrogen SCR) system 10in accordance with the invention. Partial oxidation unit 10 a receives afraction of the diesel fuel, relative to the fuel flow to engine 12,from tank 11. Partial oxidation unit 10 a may be any type of catalyst ornon-stoichiometric burner, suitable for partial oxidation ofhydrocarbons. In general, partial oxidation unit 10 a operates byconverting diesel fuel into a gas mixture containing hydrogen as one ofits primary components. In the embodiment of FIG. 1, partial oxidationunit 10 a receives diesel fuel from an auxiliary fuel line 15 off themain fuel line 13 and air from an air input line 16. An output line 17delivers the gas mixture to the main exhaust line 14.

Partial oxidation catalysts exist that can convert hydrocarbons withconversion efficiency greater than 90 percent and selectivity tohydrogen in excess of 90 percent. Certain catalysts have already beenproven effective at converting natural gas to hydrogen, namelynickel-based and rhodium-based formulations. These include Ni/Al₂O₃,Ni/La/Al₂O₃, and Rh/Al₂O₃. Although nickel-based catalysts may producecarbon, they are less expensive than rhodium-based catalysts.

Catalytic partial oxidation is a high space velocity process (e.g.,500,000 per hour), with residence times typically in the range of 10 to1000 microseconds. Thus, the catalysts do not need to be large to havehigh efficiency and selectivity. Partial oxidation catalysts operateunder reducing gas conditions, and the lambda in the partial oxidizermay be about 0.3 to 0.6.

In the embodiment of FIG. 1, an optional WGS catalyst 10 c is interposeddirectly downstream of the partial oxidation unit 10 a and upstream ofH-SCR catalyst 10 b. WGS catalyst 10 c uses carbon monoxide (CO)generated by the partial oxidation unit 10 a to form additionalhydrogen. To enable this reaction, supplemental water may be added tothe gas mixture entering WGS catalyst 10 c. An advantage of using WGScatalyst 10 c is that more hydrogen can be produced from the same amountof fuel. In other words, less fuel is needed to generate the same amountof hydrogen.

The gas mixture from WGS catalyst 10 c is injected into the main dieselexhaust line 14, upstream of H-SCR catalyst 10 b. In embodiments nothaving WGS catalyst 10 c, the gas mixture from partial oxidation unit 10a would be injected into the main exhaust line 14 at the same point. Inall embodiments, H-SCR catalyst 10 b then uses the hydrogen in the gasmixture to convert NO_(x) into nitrogen and water.

FIG. 2 illustrates a second embodiment of the invention, an H-SCR system20, whose partial oxidation unit 10 a is positioned on a branch line 22off the main exhaust line. The partial oxidation unit 10 a receives aportion of the exhaust diverted from the exhaust line, as well as dieselfuel from an auxiliary fuel line 21. Under net reducing conditions,diesel fuel is converted into hydrogen, carbon monoxide and traces ofcarbon dioxide and water. Like system 10, system 20 may have an optionalWGS catalyst 10 c downstream of the partial oxidation unit 10 a. Thehydrogen-enhanced gas mixture flows back into the main exhaust line, viaan output branch line 23, upstream of an H-SCR catalyst 10 b, which usesthe hydrogen to convert NO_(x) into nitrogen and water.

For system 20, effective partial oxidation is achieved by controllingthe diesel injection rate. When no supplemental diesel fuel is beinginjected into the exhaust stream, such as when NO_(x) emissions fromengine 12 are low, the partial oxidation unit 10 a acts as a fulloxidation catalyst, converting unburned hydrocarbons and carbon monoxideinto water and carbon dioxide. With the partial oxidation unit 10 alocated in a branch off the main exhaust gas stream, a portion of theexhaust flows through the partial oxidation catalyst. As a result, lessdiesel fuel is required to enrich the gas entering the partial oxidationcatalyst. Also, the partial oxidation catalyst can be smaller. At thesame time, sufficient hydrogen must be generated to obtain effectivereduction of the NO_(x) in the H-SCR catalyst 10 b. This design has theadvantages that the heat required to activate the partial oxidationcatalyst may be provided by the exhaust gas instead of by an externalheat source, and it may be possible to use the heat generated by thepartial oxidation reaction to accelerate heating of the H-SCR catalystlob during cold-start operation.

For both system 10 and system 20, the products of partial oxidizer 10 aare metered into the diesel exhaust gas, upstream of H-SCR catalyst 10b. The amount of gas injected should ideally be proportional to theamount of NO_(x) in the exhaust. A 1:1 molar ratio of H₂:NO is expectedfor efficient conversion of NO to N₂ in accordance with Equation (1)below. However, NO₂ exists in the diesel exhaust simultaneously with NO,either from the combustion process (approximately 15 percent) or fromoxidation in a passive particulate trap such as a CRT (approximately 40percent). A 2:1 ratio of H₂:NO₂ is expected for efficient conversion ofNO₂ to N₂ in accordance with Equation (2) below.2NO+2H₂→N₂+2H₂O  Equation (1)2NO₂+4H₂→N₂+4H₂O  Equation (2)

Results of experimentation with ruthenium-based H-SCR catalysts usingRu/MgO and Ru/Al₂O₃ have been reported by Hornung, et al. in a paperentitled “On the mechanism of the selective catalytic reduction of NO toN₂ by H₂ over Ru/MgO and Ru/Al₂O₃ catalysts”, in Topics in Catalysis,2000, 11/12 (1-4), 263-70. The reports are of 100 percent selectivity toN₂. Another possible candidate for H-SCR catalyst 10 b is a platinumtitania-zirconia catalyst, Pt/TiO₂—ZrO₂.

Potential fuel penalties may be calculated based on the NO:NO₂ ratio inthe exhaust. If a range of NO₂ content is considered from 15 to 100percent, the fuel economy penalty is calculated to be in a range fromtwo to four percent. To estimate a realistic fuel economy penalty, aworst case scenario was used with a system containing a passive PM trap,such as a CRT, which creates high levels of NO₂. Based on a 60:40 NO:NO₂exhaust gas mixture, and using Equations (1) and (2), approximately 1.4moles of H₂ are required per mole of NO_(x). Assuming ideal conditionsof 100 percent efficient partial oxidation, 100 percent selectivity toH₂, and 100 percent NO_(x) conversion efficiency of the H-SCR catalyst,it was calculated that fuel economy would be reduced by 2.5 percent.

An advantage of the invention is that the invention effectively reducestailpipe oxides of nitrogen emissions without the need for a reductantother than diesel fuel. It continuously converts NO_(x) to nitrogen, byfirst generating hydrogen from the diesel fuel and then using thehydrogen in a hydrogen-based SCR catalyst. The system does not requireadjustment of the engine air/fuel ratio, of engine combustion, or of anyother engine functionality.

Other Embodiments

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions, and alterations canbe made hereto without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A system for treating NO_(x) emissions from a diesel engine that hasat least one internal combustion chamber for combusting diesel fuel andhas a main exhaust line that carries exhaust from the engine, theexhaust having a range of exhaust gas temperatures typical for avehicular diesel engine, the system comprising: an auxiliary fuel linefrom the engine's diesel fuel supply source, separate from the main fuelsupply line to the engine; a partial oxidation unit located on theauxiliary fuel line, external to the internal combustion chamber andoff-line the main exhaust line; an air intake line for delivering air tothe partial oxidation unit; an output line for delivering ahydrogen-enhanced gas mixture from the partial oxidation unit to themain exhaust line; wherein the partial oxidation unit is operable toreceive air via the air intake line, to receive diesel fuel via theauxiliary fuel line, and to at least partially oxidize the fuel toproduce the hydrogen-enhanced gas mixture; and a ruthenium-basedhydrogen selective catalytic reduction (H-SCR) unit, located in-line onthe main exhaust line, operable to receive the hydrogen-enhanced gasmixture from the partial oxidation unit, and to use hydrogen in thehydrogen-enhanced gas mixture to continuously reduce NO_(x) emissions inthe exhaust line to nitrogen; wherein the H-SCR unit is further operableto reduce the NO_(x) emissions to nitrogen under the range of exhaustgas temperatures; means for determining the amounts of NO and NO₂ in theexhaust gas; and a metering device on the output line for maintaining aratio of the hydrogen to NO_(x) in the main exhaust line, on the basisof a desired ratio of the hydrogen to NO and a desired ratio of thehydrogen to NO₂.
 2. The system of claim 1, wherein the partial oxidationunit is a non-stoichiometric burner.
 3. The system of claim 1, whereinthe partial oxidation unit has a catalyst.
 4. The system of claim 3,wherein the partial oxidation unit is a nickel-based catalyst.
 5. Thesystem of claim 3, wherein the partial oxidation unit is a rhodium-basedcatalyst.
 6. The system of claim 1, wherein the partial oxidation unitis a combination of a non-stoichiometric burner and a catalyst.
 7. Thesystem of claim 1, further comprising a water gas shift catalystdownstream of the partial oxidation unit and upstream the catalyticunit, operable to generate further hydrogen from the gas mixture.
 8. Thesystem of claim 1, wherein the H-SCR unit comprises ruthenium andmagnesium.
 9. The system of claim 1, wherein the H-SCR unit comprisesruthenium and aluminum.
 10. A system for treating NO_(x) emissions froma diesel engine that has at least one internal combustion chamber forcombusting diesel fuel and has a main exhaust line that carries exhaustfrom the engine, the exhaust having a range of exhaust gas temperaturestypical for a vehicular diesel engine, the system comprising: anauxiliary fuel line from the engine's diesel fuel supply source,separate from the main fuel supply line to the engine; a partialoxidation unit located on the auxiliary fuel line, external to theinternal combustion chamber and off-line the main exhaust line; anexhaust gas intake line for delivering a portion of the engine exhaustgas from the main exhaust line to the partial oxidation unit; an outputline for delivering a hydrogen-enhanced gas mixture from the partialoxidation unit to the main exhaust line, downstream the exhaust gasintake line; wherein the partial oxidation unit is operable to receivethe portion of the engine exhaust via the exhaust gas intake line, toreceive diesel fuel via the auxiliary fuel line, and to at leastpartially oxidize the fuel to produce the hydrogen-enhanced gas mixture;a ruthenium-based hydrogen selective catalytic reduction unit, locatedin-line on the main exhaust line operable to receive thehydrogen-enhanced gas mixture from the partial oxidation unit, and touse hydrogen in the hydrogen-enhanced gas mixture to continuouslyconvert the NO_(x) emissions in the exhaust line to nitrogen; whereinthe H-SCR unit is further operable to reduce the NO_(x) emissions tonitrogen under the range of exhaust gas temperatures; means fordetermining the amounts of NO and NO₂ in the exhaust gas; and a meteringdevice on the output line for maintaining a ratio of the hydrogen toNO_(x) in the main exhaust line, on the basis of a desired ratio of thehydrogen to NO and a desired ratio of the hydrogen to NO₂.
 11. Thesystem of claim 10, wherein the partial oxidation unit is anon-stoichiometric burner.
 12. The system of claim 10, wherein thepartial oxidation unit is a catalyst.
 13. The system of claim 10,wherein the partial oxidation unit is a nickel-based catalyst.
 14. Thesystem of claim 10, wherein the partial oxidation unit is arhodium-based catalyst.
 15. The system of claim 10, wherein the partialoxidation unit is a combination of a non-stoichiometric burner and acatalyst.
 16. The system of claim 10, further comprising a water gasshift catalyst downstream of the partial oxidation unit and upstream thecatalytic unit, operable to generate further hydrogen from the gasmixture.
 17. The system of claim 10, wherein the H-SCR unit comprisesruthenium and magnesium.
 18. The system of claim 10, wherein the H-SCRunit comprises ruthenium and aluminum.
 19. The system of claim 10,wherein the exhaust gas intake line is a bypass line off the mainexhaust line.